Optical depolarizers have found applications in two fields, the test and measurement as well as the design of Raman amplifiers. Different existing designs of fiber depolarizers have been documented. The most popular is the Lyot depolarizer, based on polarization maintaining fiber (PMF), such depolarizer is disclosed, for example, by J. Noda, K. Okamoto and Y. Sasaki, in J. Lightwave Technology 4, 1071-1089 (1986). However, for narrow-band sources this approach is not an option because of the long lengths of PMF required.
Polarization scrambling based on a directional coupler with a fiber ring structure and polarization controllers has also been described. This structure scrambles a well known input state of polarization (SOP) as disclosed, for example, by P. Shen and J. C. Palais, in Opt. Fiber Technology 3, 184-188(1997). The sensitivity to the input polarization fluctuation is a limiting factor to the stability of such depolarizer. A cascade of directional couplers allows an all SOP depolarizer, increasing the stability to the detriment, however, of insertion loss as disclosed, for example, by P. Shen and J. C. Palais, in Opt. 3, 1686-1691 (1999).
The present applicant has also described and claimed an all-fiber depolarizer in Canadian patent application No. 2,357,955 and U.S. patent application Ser. No. 10/045,190 which are incorporated herein by reference. In this depolarizer, a beam splitter having two input fibers and two output fibers is used and polarized light is controllably injected into one of the input fibers of the beam splitter, so that the polarization of the signal entering the beam splitter is at a 45° angle from the polarizing axis, and a loop is formed between the second input fiber and one of the output fibers of the beam splitter, said loop being made of a standard non-birefringent fiber and having a length greater than the coherence length of the light source. One of the embodiments disclosed in this prior patent application and illustrated in FIG. 3 thereof provides for a design based on a Mach-Zehnder interferometer structure (MZ) with a polarization maintaining fiber (PM) as a half wave plate on one of the MZ branches. A fiber ring delay line is also formed by the MZ by connecting one of the output ports to one of the input ports. The principle of operation is similar to the depolarizer based on a directional coupler and the Lyot depolariser.
The DOP of light of the MZ with a fiber ring delay line can be written as:   DOP  =            [              1        -                  4          ·                                                    ∑                                  k                  =                  0                                ∞                            ⁢                                                ∑                                      j                    =                    0                                    ∞                                ⁢                                  (                                      1                    -                                                                  g                        ⁡                                                  (                                                                                    (                                                              k                                -                                j                                                            )                                                        ·                            τ                                                    )                                                                    ·                                              lx                        k                                            ·                                              ly                        j                                                                              )                                                                                    ∑                                  m                  =                  0                                ∞                            ⁢                                                (                                                            lx                      m                                        +                                          ly                      m                                                        )                                2                                                        ]              1      /      2      where Ix and Iy are light intensities of X and Y polarizations, g is the Fourier transform of the normalized spectral shape of the source, k, j and m indicate the number of circulations in the fiber ring delay line and τ is the time delay between the two polarizations. The delay line structure works as a depolarizer for non-interferometric operation condition. In this condition each recirculating beam is noncoherent with the other beams. This is verified when the length of the fiber ring delay line is much longer than the coherence length of the light source. In this case:       g    ⁡          (                        (                      k            -            j                    )                ⁢        τ            )        =                                  1                          if                                      k            =            j                                                0                          if                                      k            ≠            j                                  
The design described above works as a depolarizer if condition of equal power on the orthogonal states of polarization X/Y of the MZ-PM fiber is verified. Minimum DOP is achieved for an input azimuth of 45°. The DOP is dependant on the MZ loss and isolation as well as the circulating ring. In the ideal case double losses are induced for the X polarization. Like the fiber ring delay line with one directional coupler, this design is subject to DOP and loss variations when temperature varies. To avoid these fluctuations, the polarization must be maintained over its propagation without being affected by temperature.
The stability of the DOP is a key parameter for industrial applications. Thus, the input SOP must be carefully tuned to ensure a low DOP, and the splice quality between the device and the PM-output light source must be as high as possible (typically 30 dB of extinction ratio). In addition, the fluctuation of the SOP in the fiber ring must be maintained over all the environmental conditions specified. These conditions are difficult to meet in practice.
There is thus a need for an improved all-fiber depolarizer that would obviate the above problems.